1. Field of the Invention
The present invention relates to circuitry for affecting the transmission characteristics of a voice frequency improving device such as a repeater and more particularly to circuitry which on sensing a tone of a predetermined frequency and predetermined duration disables the normal operation of the repeater to thereby permit transmission of data through the repeater.
2. Description of the Prior Art
In communication systems wherein voice signals are transmitted over substantial distances through transmission lines, it is necessary to provide circuitry which can compensate for the attenuation of the signals by the transmission line. In telephone systems, for example, it is necessary to provide amplifier circuits or repeaters to maintain satisfactory signal transmission through telephone lines which, in the absence of such circuits, would excessively attenuate the signals transmitted therethrough.
In the development of circuitry for transmitting voice frequency signals over transmission lines, a variety of types of repeater circuits have been employed. One of these types of repeater circuits is a repeater having a series amplifying network for inserting, in series with the transmission line, an amplifying voltage which varies in accordance with the signal voltage across the transmission line and a shunt amplifying network for inserting, in shunt with the transmission line, an amplifying current which varies in accordance with the signal current through the transmission line. The amplifying voltage is inserted in the transmission line in aiding relationship to the signal voltage transmitted by the then dominant or louder talking party and in opposing relationship to the then nondominant or softer talking party. The amplifying current is inserted in the transmission line in aiding relationship to the signal current transmitted by the then dominant or louder talking party and in opposing relationship to the then nondominant or softer talking party. A direction detector is connected to the transmission line in a manner such that the dominant direction of transmission is determined to thereby insert the voltage and current to aid the dominant party.
In such repeaters, it may be shown that if the ratio of amplifying voltage to signal voltage is equal in magnitude, but opposite in sign to the ratio of amplifying current to signal current, the circuit may function as an impedance matching circuit. It may also be shown that if the above ratios are equal in magnitude and have the same sign, then the circuit may function as a repeater which compensates for the frequency independent attenuation of loaded transmission lines or the frequency dependent attenuation of nonloaded transmission lines. One such repeater which compensates for the attenuation of signals in a loaded transmission line is shown and described in U.S. Pat. No. 3,706,862 granted in the name of C. W. Chambers, Jr. on Dec. 19, 1972. A repeater which compensates for the attenuation of signals in a non-loaded transmission line is shown and described in U.S. Pat. No. 3,818,151 granted in the name of C. W. Chambers, Jr. et al on June 18, 1974. Repeater circuits of these types are referred to as amplifying or switched gain type repeaters.
Where it is desired to transmit data rather than voice over a transmission line utilizing such a repeater, it is necessary to disable the repeater prior to the data transmission. The repeater must be disabled in order that the detrimental effect the direction detector has on data transmission may be minimized. Without such disablement the direction detector would slow the transfer of data due to the time the detector circuitry takes to determine the dominant and nondominant parties. Additionally by placing the repeater in a disabled or no gain state during data transmission the repeaters function of providing simultaneously gain to the dominant party and loss to the non-dominant party will be avoided.
The device from which the data is to be transmitted is typically connected to the telephone transmission line by a modulator-demodulator (modem). Prior to the transmission of the data the modem causes a tone of predetermined frequency and duration to be transmitted on the line. The specifications of the Bell operating companies provides, for example, that the tone be in the frequency band of 2,000 Hz to 2,250 Hz and have a duration of at least 400 milliseconds. The tone is usually transmitted in the frequency range of 2,010 Hz to 2240 Hz. Upon sensing of the tone signal, the repeater becomes disabled and remains so disabled during the data transmission until the transmitted signal level falls below a predetermined amplitude for a predetermined interval of time. Upon the termination of data transmission the repeater is enabled to thereby provide simultaneously amplification to the dominant voice frequency party and loss to the nondominant voice frequency party. Thus a data tone responsive disabling circuit must be included in the repeater in order that the repeater be disabled and remain disabled during data transmission.
Data tone responsive disabling circuitry may also be used in conjunction with devices such as echo suppressors which are also used to improve the quality of voice transmission. Echo suppressors are used on those voice communication circuits which have appreciable propagation delay. Echo is suppressed by inserting additional loss in the transmission path to thereby prevent a return echo to the talking party. Echo suppressors utilize data tone disabling circuitry to inhibit the loss insertion circuit to thereby render the echo suppressor transparent to the transmission of data. Thus a data tone responsive disabling circuit must be included in an echo suppressor in order that the suppressor be disabled and remain disabled during data transmission.
One such prior art data tone responsive disabling circuit is that used by Lenkurt Electric Co., Inc. in the Model 931C echo suppressor. The Lenkurt disabler uses a first L-C circuit combination to provide a maximum output voltage if the transmitted signal lies within the tone signal frequency band. A second L-C circuit combination is used to provide a maximum output voltage if the transmitted signal lies outside of the tone signal frequency band. The first and second L-C circuit combinations function, in effect, as band pass and band reject filters, respectively. A Schmitt trigger circuit is used in conjunction with the L-C circuit combinations to provide a signal which disables the loss insertion circuit upon detection of a tone signal.
Another such prior art data tone responsive disabling circuit is that used by Tellabs, Inc. in their Model 7201 repeater. The disabling circuit used therein includes an L-C parallel resonant circuit which functions as a band pass filter for the tone signal. The inductor used in the band pass filter is realized by active filter techniques in that a gyrator circuit is used to simulate the needed value of L. A limiter circuit provides a constant level input signal to the filter. Disabling occurs when the signal output from the filter exceeds the amplitude of those signals which are outside of the filter's band width by a predetermined value.
While each of the circuits described above provides a disabling function for its associated echo suppressor or repeater, it was recognized that inherently more reliable disabling circuitry could be provided by use of the present invention. The present invention utilizes tone signal detection and capture circuitry such as a phase locked loop which provides both a high degree of immunity insuring that signals which are outside of the tone frequency band will not be captured and a characteristic which insures that even varying amplitude signals in the tone frequency band will be captured. The prior art circuits may not necessarily offer the immunity to recognizing signals outside of the tone frequency band as the tone signal and the relative insensitivity to variations in tone signal amplitude that is offered by the present invention. The circuit of the present invention further initiates tone detection only after first determining that the transmission line signal does not contain high amplitude voice frequency signals lying within a critical predetermined band of frequencies.